J. Even Ø. Nilsen, Helge Drange, Kristin Richter, Eystein Jansen and Atle Nesje :
Changes in past, present, and future sea level on the coast of Norway a project by Nansen Environmental and Remote Sensing Center and UNI Research, at the Bjerknes Centre for Climate Research, funded by the City of Bergen, Department of Urban Development, Climate, and Environmental A airs.
NERSC Special Report no. 89 Bjerknes Centre for Climate Research publication no. R101
Bergen, August 2012 This is NERSC Special Report 89, publication no. R101 from the Bjerknes Centre for Climate Research.
The main parts of this report are to be cited as follows.
Chapter 4 is to be cited as: Jansen, E. (2012). Paleoclimatic perspectives on sea level. In Nilsen, J.E.Ø., H. Drange, K. Richter, E. Jansen, A. Nesje. Changes in the past, present, and future sea level on the coast of Norway. NERSC Special Report 89, Bergen, Norway.
Chapter 5 is to be cited as: Richter, K., J.E.Ø. Nilsen, H. Drange (2012). Contributions to observed sea level change for1960-2010. In Nilsen, J.E.Ø. , H. Drange, K. Richter, E. Jansen, A. Nesje. Changes in the past, present, and future sea level on the coast of Norway. NERSC Special Report 89, Bergen, Norway.
Chapter 6 is to be cited as: Drange, H., J.E.Ø. Nilsen, K. Richter, A. Nesje (2012). Updated estimates of future sea level rise on the Norwegian coast. In Nilsen, J.E.Ø., H. Drange, K. Richter, E. Jansen, A. Nesje. Changes in the past, present, and future sea level on the coast of Norway. NERSC Special Report 89, Bergen, Norway.
The published article in Appendix 2 is to be cited as: Richter, K., J.E.Ø. Nilsen, H. Drange (2012). Contributions to sea level variability along the Norwegian coast for 1960-2010. J. Geophys. Res., 117, doi:10.1029/2009JC007826.
The full report is to be cited as: Nilsen, J.E.Ø., H. Drange, K. Richter, E. Jansen, A. Nesje. (2012). Changes in the past, present, and future sea level on the coast of Norway. NERSC Special Report 89, Bergen, Norway. 48 pp.
City of Bergen, Department of Urban Development, Climate, and Environmental A airs: www.bergen.kommune.no/byutvikling Nansen Environmental and Remote Sensing Center: www.nersc.no Uni Research: www.uni.no Bjerknes Centre for Climate Research: www.bjerknes.uib.no University of Bergen: www.uib.no MARE: www.mare-project.eu August 2012
Changes in the past, present, and future sea level on the coast of Norway1
Project report to the City of Bergen, Department of Urban Development, Climate, and Environmental Affairs
1,2 3,2 4,2 5,4,2 5,2 Jan Even Øie Nilsen , Helge Drange , Kristin Richter , Eystein Jansen and Atle Nesje 1 Nansen Environmental and Remote Sensing Center, Bergen 2 Bjerknes Centre for Climate Research, Bergen 3 Geophysical Institute, University of Bergen 4 Uni Research AS, Bergen 5 Department of Earth Science, University of Bergen
Sea levels are rising, predominantly due to the warming of the oceans, melting of land- based ice, and ground water depletion. In addition land surfaces rise and sink. The west coast of Norway is still rising after the retreat of the Fennoscandian ice sheet of the last ice. Presently, the rates of ocean and land rise are comparable, but under global warming the sea levels on the Norwegian coast are expected to rise by 20 to 80 cm by the end of the century. In 50 years about half of this rise is estimated. In the latter half of this century the expected sea level rise will impose increased challenges upon existing infrastructure, and adapting plans for new infrastructure to an ever-rising sea level can be advantageous. In this research project, changes in sea level in prehistoric times as well as during the latest 50 years are studied, the state of the present sea level is assessed, and updated estimates for sea level rise in the 21st century is presented.
1 To be cited as Nilsen, J.E.Ø., H. Drange, K. Richter, E. Jansen, A. Nesje (2012). Changes in the past, present, and future sea level on the coast of Norway. NERSC Special Report 89, Bergen, Norway. 48 pp.
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Table of Contents 1. Summary...... 3 2. Administrative details...... 4 3. Introduction...... 4 4. Paleoclimatic perspectives on sea level ...... 5 Mean sea level ...... 5 Rates of change ...... 6 5. Contributions to observed sea level change for 1960-2010...... 8 Methods ...... 9 Results...... 10 Discussion...... 12 Conclusions ...... 13 6. Updated estimates of future sea level rise on the Norwegian coast ...... 14 Background ...... 14 Observed sea level rise...... 15 Global sea level in the future...... 15 Regional sea level in the future ...... 17 7. Dissemination ...... 21 Bibliography ...... 22 Appendix...... 24 Appendix 1: Estimates of future sea level rise for the Norwegian coastal municipalities...... 24 Appendix 2: Peer review publication on sea level change during the past 50 years .31
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1. Summary The estimated global sea level rise for the two recent decades is 3 mm/yr, twice as fast as the average rise throughout the last century. We know the surface of the oceans will continue to rise for a long time into the future, even hundreds of years after humanity learns how to control greenhouse gas emissions. In this project, prehistoric sea level and rates of change have been assessed, the mechanisms involved in sea level rise have been studied based on observations from modern times, and future sea level rise has been estimated based on the current knowledge. About 3 million years ago, when the continents were already in today's positions, the climate on Earth was significantly warmer than today, the large ice sheets of Greenland and Antarctica were smaller and the sea level 10-30 m higher. During the last interglacial (about 120.000 years ago) global temperature was about 1-2°C warmer than today. The sea level was 4-10 m higher, mainly due to less water stored as ice on land and the thermal expansion of the oceans. The sea level change during the last interglacial was around 2 mm/yr, which is comparable to the presently observed rates. If today's ice sheets of Greenland and West-Antarctica were to become unstable and partly collapsing, sea level rates may become similar to those found after the ice ages. These rates have been estimated to be up to 40 mm/yr at certain locations. In modern times, the different processes affecting regional sea level change can be studied using various observations. In the study focussing on the Norwegian coast, it is shown that short-term changes in local sea level are to a large extent caused by changes in temperature (thermal expansion) and salt content (haline contraction) of seawater, as well as changes in local atmospheric pressure. In contrast, less than half of the observed long-term changes (the trend) can be explained by these processes and land uplift. In fact, apart from the land uplift, only thermal expansion contributes to a significant trend along the entire Norwegian coast. The observed trend in relative sea level (the sea level observed from shore) is 0.9 mm/yr in Bergen in the period 1960-2010. For the absolute sea level (i.e. without compensating for land uplift) the rise would be 2.6 mm/yr. Of this 0.9 mm/yr can be attributed to thermal expansion, and 0.7 mm/yr is estimated to be due to melting land ice. The remainder is subject to different processes with large uncertainties, and further research is necessary to accurately quantify their importance. Future sea level rise can be estimated by combining sea level due to changes in the oceans' temperature, salt content and circulation as projected by climate models, with estimated contributions from land ice and water stored on land, changes in the gravity field, redistribution of sea water within the oceans and vertical land movement. Based on estimates of future global sea level rise and by taking into account the aforementioned processes and standard estimates for uncertainties we estimate that the sea level in Bergen will be between 20 to 80 cm higher within 100 years, with a probability of 66%. The data needed to compute these numbers are taken from the latest available literature. The computation of the likely sea level change has been conducted for all coastal communities in Norway (Figure 5 and Table 4). Due to the increasing amount and higher accuracy of observations and a constantly increasing understanding of the processes affecting sea level, projections of global and local sea level rise should be updated regularly.
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2. Administrative details Project: Project number M 760, Sub-project M780: Changes in the past, present, and future sea level on the coast of Norway. The project is a sub-project under MARE. Administrative responsible: Prof. Ola. M. Johannessen, Nansen Environmental and Remote Sensing Center (NERSC) Prof. Eystein Jansen, Bjerknes Centre for Climate Research (Uni Research AS) Research leadership: Dr. J. Even Ø. Nilsen (NERSC) Prof. Helge Drange (University of Bergen) Contract partner: Nansen Environmental and Remote Sensing Center Thormøhlensgt. 47 N-5006 Bergen Norway Duration: May 2009 - May 2012
3. Introduction Tide gauges around the world show that global sea level has risen by about 17 cm during the past 100 years, and it appears that the sea level rise is accelerating. Satellite measurements indicate a rise of 3 mm/yr since 1993 or almost twice as much as the average rise during the past century. It is also well known that the rise will continue even after CO2 emissions will be stabilized. It is therefore necessary to adapt and be prepared to rising sea levels. The degree to which measures should be taken in the present and future to mitigate the consequences of rising seas depends on the risk assessment. This is not addressed in this report. The projection of sea level rise in the future is subject to large uncertainties. These uncertainties arise both from uncertainties in future greenhouse gas emissions as well as uncertainties in the relative contributions from the different processes leading to changing sea levels. E.g. how quickly the ocean is warming, how fast the ice on land melts, what is the rate of vertical land movement, and how is Earth's gravity field affected? The structure of the project has been as follows: Prehistoric data has been studied to assess previous sea levels, as well as constraining possible rates of change; the instrumental record, the present, has been used to study sea level changes along the Norwegian coast and its causes during the past 50 years; projections of future sea level have been computed using a combination of published data and increased understanding of the individual contributions.
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4. Paleoclimatic perspectives on sea level 2 E. Jansen Due to the commonly existing sea level changes in Earth´s history, present and future changes might be placed into a longer time perspective from knowledge about past sea level changes. Of interest here are both the absolute values of sea level changes in past warm periods of Earth´s history and the rates of change experienced during previous periods of rapid sea level change. Sea level in previous periods can be calculated from data that reconstructs the position of earlier shorelines, sedimentary basins which change character from marine to lake conditions, or through the utilisation of fossils from organisms that have a known habitat at the sea surface, e.g. corals, and therefore can trace the mean sea level of the time they lived. When it is possible to date the mean sea level position in situations such as these in a specific locality, one needs to further correct sea level estimates to modern sea levels due to vertical movements of the land surface due to isostatic movements or tectonic factors that have changed the position of the locality. Such corrections can in many places be made with high degree of certainty, hence it is possible to establish both the absolute sea level of past times and, in some situations, also the rates of change of past sea level variations. Mean sea level Due to the importance of sea level in the context of global warming, a high number of new studies of past sea levels have been performed during recent years, and a growing body of scientific literature sheds light on these changes. Sea level in the Pliocene epoch During the Pliocene era (3-5 million years before present) the mean climate state of the Earth was over long periods (more than 100.000 years) significantly warmer than present. At the same time the CO2-content of the atmosphere is estimated to have been 400-420 ppmv (parts per million by volume), i.e. not significantly higher than the concentration will be in one or two decades from now. This period is not so far back in time and the surface of the Earth was quite similar to present in terms of the positioning of continents and size and place of mountain ranges. Since the warm periods of the Pliocene had such a long duration, it is reasonable to assume that the slow acting elements of the climate system were in equilibrium during this period, which therefore can provide us with estimates of future sea level on the longer-term with a slow and steady adjustment to the existing boundary conditions. Recent literature indicates that Pliocene sea levels were 20 ± 10 m above modern sea levels, with a 66% likelihood that it was between 12 and 30 higher than today, based on a set of 34 globally distributed locations. These estimates are slightly lower than earlier estimates (which were up to 40 m higher than today; Brigham-Grette and Carter, 1992). The adjustment to lower estimates is due to more precise modelling of the movements of the Earth's crust at the locations. These results imply that the land-ice volume in Antarctica and Greenland were significantly less
2 To be cited as Jansen, E. (2012). Paleoclimatic perspectives on sea level. In Nilsen, J.E.Ø., H. Drange, K. Richter, E. Jansen, A. Nesje. Changes in the past, present, and future sea level on the coast of Norway. NERSC Special Report 89, Bergen, Norway.
5 August 2012 in the Pliocene than now, and that the West-Antarctic ice-sheet probably was not existing during periods of this time (Naish et al., 2009; Pollard and DeConto, 2009). The last interglacial The last interglacial period, approximately 130.000-120.000 years before present, was a period with mean global temperature 1-2oC above late 20th century temperatures, and a significantly enhanced warming in polar regions. At this time the Earth's orbit brought the Earth closer to the sun during Northern Hemisphere summer than now with a slightly warmer climate as a consequence. Atmospheric CO2 levels were, however, at 280-300 ppmv, i.e. significantly lower than they are now. There are extensive data sets available to document that mean sea levels were substantially higher than now during the last interglacial. The newest estimates indicate that sea level was at least 6 m higher than now (with an uncertainty interval spanning 4 to 10 m above modern sea level), based on lifted marine terraces and the oxygen isotope composition of the sea (which change when global land-ice volume changes) (Kopp et al., 2009; Rohling et al., 2009; Lisiecki and Raymo, 2005). Both Greenland and West Antarctic ice sheets were smaller than now, likely 30% smaller in the case of Greenland. Estimates of the steric effect of the warmer ocean indicate at most 0.3 m higher sea level from this effect (McCay et al., 2011). Most of the sea level rise must therefore be ascribed to deglaciation of the ice sheets. It is difficult to discriminate between Greenland and West Antarctica, and this issue is an object of intensive research. These results show, however, that polar ice sheets and associated sea levels are highly sensitive to increasing temperatures. Present interglacial period (the Holocene) After sea levels rose about 120 m at the end of the last glaciation, ending at about 6000 years before present, due to the demise of the large continental ice sheets, global sea levels have been quite stable. During the last 5000 years sea levels have been relatively constant. Some long-term adjustments of Antarctic ice sheets have been registered to last as long as up to 3000 years before now, hence one can only compare present sea levels to historic sea levels in detail over the last 2000 years. Our methods to reconstruct sea levels for this period have an accuracy of about 20 cm. Until approximately 1900 AD sea levels fluctuated within a range of 0 to 20 cm, while after 1900 AD these fluctuations were replaced by a monotonous rise to modern sea levels. Rates of change The accuracy of dating past sea levels is too low to allow estimates of rates of change for older times than the last interglacial. A number of studies estimate how fast sea level rose up to the highest recorded levels of the last interglacial. These indicate a maximum rate of change of 1.6 m/100 years (Rohling et al., 2009), but the results are disputed. Other estimates conclude with maximum rates 1/10 of this, i.e. 20 cm/100 years (Blanchon et al., 2009). The present day rate of change is, for comparison, 30 cm/100 years. During deglaciations at the end of the ice ages, large continental ice sheets collapsed, and the rates of change of sea level during these most extreme periods of sea level rise may indicate an upper boundary for how fast sea level may rise in a situation where the Greenland and West Antarctic ice-sheets may enter into a similar situation of collapse. Reconstructed rates of change of up to 4 m/100 years have been estimated for shorter periods of the last deglaciation rate (Bard et al., 1990; Hanebuth et al. 2000). It is difficult to provide global estimates during these periods due to many local imprints, but it is
6 August 2012 reasonable to believe that the rates may have been at least 2 m/100 years. Such a rate can be extracted from the reconstruction from Sotra by Lohne et al. (2007; Figure 1). After extracting the imprint from isostatic rise of the land surface due to the disappearance of the glacial ice load, the mean sea level rise for the initial phase of the deglaciation was approximately 60 cm/100 years.
Figure 1: Relative sea level curve for Sotra, based on studies of lake sediments from isostatically raised lakes, during the past 14.500 years. Crosses represent radiocarbon dates with uncertainty estimates in time and height, which the curve is drawn through. The dotted line is drawn through the actual dates, while the stippled line is considered more accurate due to probable dating errors imposed by the Storegga-tsunami 8.100 years ago. Figure is from Lohne et al. (2007).
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5. Contributions to observed sea level change for 1960-20103 K. Richter, J.E.Ø. Nilsen, H. Drange Global sea level has been rising by about 20 cm during the last century and is expected to continue to rise in the 21st century. The rise and variability is not spatially uniform. To be able to project local changes in relative sea level (RSL), it is important to identify the processes that govern regional RSL variability. In this study, we assess the importance of different contributions to RSL variability along the coast of Norway in the period 1960– 2010. This work is published as a research article in Journal of Geophysical Research (Richter et al., 2012; Appendix 2). The following is a summary of the work and the most important results. The relative importance of the different factors that contribute to changes in global, and local, sea level varies with time. According to the Fourth Assessment Report of the Intergovernmental Panel on Climate Change (IPCC), thermal expansion contributed with about 40% while glaciers and ice sheets accounted for the remaining 40% and 20%, respectively, in the period 1961–2003 (Meehl et al., 2007). For 2003-2007 thermal expansion and ice sheets have been estimated to contribute 10% and 40%, respectively (Cazenave and Llovel, 2010). The individual contributions also vary geographically, that is their magnitude may be different in different parts of the globe. The focus of this study is on the Norwegian coast and the following effects are investigated: the expansion and contraction of sea water due to changes in temperature and salt content, changes in air pressure and the effect of land uplift. Changes in the heat (temperature) and salt (salinity) content of sea water result in thermal expansion and haline contraction. The resulting change in sea level is referred to as change in steric height. Warmer water expands while more salt leads to contraction of seawater. In the world oceans the temperature effect dominates, but in the cold waters of the Nordic Seas and Arctic Ocean changes in salinity are at least as important as temperature effects. Steric height variations alter the sea level by changing the volume of seawater through expansion and contraction in contrast to adding water to or removing it from an ocean region. The latter can be due to melting land ice (glaciers and ice sheets) or moving of water masses from one ocean region to another. Changes in atmospheric pressure over the oceans will cause sea water to move and therefore change the local sea level. A low-pressure system means less air masses (less weight) and the ocean surface will respond by rising. The water necessary to do so will come from areas of high air pressure where more weight depresses the ocean surface. This way, air pressure changes move water from areas of high atmospheric pressure to low atmospheric pressure. As a rule of thumb, a 1 mbar decrease in air pressure results in a 1 cm rise in sea level. In addition, a low-pressure system (a storm) is usually accompanied by winds that, through friction, get the seawater moving. The Norwegian coast is commonly subject to incoming storms from the southwest that are pushing water towards the shore. The results are storm surges for isolated storms and long-term increased sea level for a continuous stream of storms from the southwest.
3 To be cited as Richter, K., J.E.Ø. Nilsen, H. Drange (2012). Contributions to observed sea level change for 1960-2010. In Nilsen, J.E.Ø., H. Drange, K. Richter, E. Jansen, A. Nesje. Changes in the past, present, and future sea level on the coast of Norway. NERSC Special Report 89, Bergen, Norway.
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Land uplift is an important factor when assessing sea level changes. It is the sea level with respect to the shore that is of major interest for adaptation and mitigation. In addition direct measurements of sea level are being carried out from land by tide gauges. Therefore, we separate between absolute sea level (sea surface height, SSH) and relative sea level (RSL), where the latter is relative to the shore. The land uplift in Norway is mainly due to the complete melting of the Fennoscandian ice sheet that covered Scandinavia during the last ice age. The Earth is still adjusting to the removal of the weight of the ice masses (glacial isostatic adjustment, GIA) and, in the period this study is concerned with, the effect can be described as a constant uplift. Methods In this study, we consider changes induced by atmospheric and thermohaline variability, as well as vertical land uplift. The combination of these contributions yields the reconstructed RSL,
RSLrc = ηp +ηT +ηS + GIA , (1)
which will be compared to the observed RSL. In the expression above, ηp is the SSH variability due to surface air pressure fluctuations, ηT and ηS are the thermosteric and € halosteric contributions, respectively, and GIA is a linear trend representing vertical land uplift due to glacial isostatic adjustment. Accordingly,
RSL = RSLrc +ηres, (2)
where ηres is the sea level residual that is not explained by our reconstruction.
72 o € N
Ingøy Honningsvåg
Hammerfest
68 o N Harstad Tromsø Eggum Narvik Kabelvåg Skrova Bodø
64 o N Rørvik
Bud Heimsjø Kristiansund Ålesund Måløy 60 o Sognesjøen N Bergen Oslo Ytre Utsira Indre Utsira Stavanger Lista Tregde o 56 N
o o o 0 10 E 20 E Figure 2: Positions of the tide gauges used in the analysis (black) and locations of the hydrographic stations (blue). The observed RSL is obtained from tide gauges along the entire Norwegian coast (Figure 2). Due to the limited length of the available records the study focuses on the period 1960-
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2010. We use monthly data in our analysis and perform the reconstruction according to equation 1 for each individual tide gauge. Atmospheric pressure is obtained from the reanalysis data of the National Centers for Environmental Prediction-National Center for Atmospheric Research (NCEP-NCAR). The data is from an atmospheric model that includes information from actual meteorological observations (including local air pressure) and provides a consistent estimate of atmospheric variables (pressure, wind, etc.) at positions in between the meteorological stations. The spatial resolution is 2.5 degrees and we use the pressure from the ocean grid point closest to each tide gauge. For components of steric height we use hydrographic station data along the Norwegian coast provided by the Institute of Marine Research (IMR), Bergen, Norway. There are eight permanent stations along the Norwegian coast (Figure 2) that have been maintained for several decades and provide vertical profiles of temperature and salinity throughout the year and through the whole water column. The locations of the hydrographic stations are not identical to the locations of tide gauges (Figure 2). Therefore, RSL observations from tide gauges have been paired with the steric height based on their location and the highest correlation coefficients between steric height and RSL observed with tide gauges (see Appendix 2 for details). Land uplift (GIA) can be estimated in different ways by including geodetic datums (reference points on land), global positioning system (GPS) measurements, geodynamic modelling, or sea level observations. The estimates of the group around Peltier (Peltier, 2004) are widely used. However, they apply a global Earth model, such that small-scale anomalies in Earths structure are not properly modelled and local uplift rates cannot be expected to be accurate. We therefore chose to use uplift rates by Vestøl (2006) who combined levelling (geodetic datums), historical tide gauge recordings, and GPS data to derive land uplift rates for Fennoscandia. In the meantime, a new and improved data set has been released by Simpson et al. (2012). We will discuss our results with respect to the new rates and discuss the differences in the next section. Results The goal of the study was to assess how well the combination of the chosen components represents the observed sea level trend and variability for the past 50 years. Covariance, that is the common variability on time scales shorter than the period that is studied, reveals the direct influence different processes have on observed sea level. This study shows that the three contributions (thermal expansion, haline contraction and atmospheric pressure changes) are responsible for 70-85% of the observed sea level variability at all positions except the two southernmost stations (Tregde and Oslo). Land uplift is not considered in the covariance analysis, as it is a constant trend. For Bergen, 76% is explained. These are high numbers when taking into account that all data are also subject to random noise. The high numbers show that a large part of the variability is explained by our relatively simple reconstruction (equation 1). For Bergen, 46% of the observed RSL variability is explained by the pressure effect, 29% by the thermosteric effect (sea temperature) and 34% by the halosteric effect (salt). As these contributions are not completely independent from each other, their combination explains less than the sum of the individual components. The explained variances are somewhat reduced when using yearly data indicating that other processes become more important on longer time scales.
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Honningsvåg Hammerfest Tromsø Harstad Narvik Kabelvåg Bodø Rørvik Heimsjø Kristiansund Ålesund RSL Måløy Bergen SSH Stavanger SSH new Tregde Oslo
−2 −1 0 1 2 3 4 −1 trend (mm yr ) Figure 3: Trends in the sea level at different locations on the Norwegian coast, with error bars. In black is the relative sea level (RSL) directly from tide gauge records, in red is our estimate of absolute sea level rise (i.e. corrected for land uplift), while in green is absolute sea level rise using the land uplift estimates from Simpson et al. (2012).
The trends in sea level during the period 1960-2010 vary geographically (Figure 3). This is in particular true for the RSL trends (the trends observed from the shore) and is due to the uplift rates being very different from one station to another. The uplift rate at a given point depends on the distance of that point to the centre of mass of the Fennoscandian ice sheet, the Gulf of Bothnia. The RSL trends are positive from Tregde to Ålesund and north of Harstad, and negative or close to zero elsewhere. After correcting for land uplift and computing the SSH trends, the differences are reduced and the trends are positive and larger than 1.7 mm/yr at all stations (the mean in Figure 3 is 2.6 mm/yr). When using the new uplift rates by Simpson et al. (2012), the SSH trends become larger but stay within the uncertainty intervals of our estimates.